In the adult, blood cell production or haemopoiesis takes place mainly in the bone marrow. The blood cell types produced are a reflection of the needs of the organism at any moment, for example bacterial infection leads to a large increase in neutrophil production. The rate and scale of blood cell production in vivo are regulated, at least in part, by the synthesis and release of specific cytokines both within the bone marrow and also from other tissues. Here we detail the range of cytokines which act directly on haemopoietic stem cells and myeloid progenitor cells. Also cellular systems which will permit the elucidation of the specific interactions between these various cytokines which regulate stem cell self-renewal, differentiation and proliferation are described.